The Internal Revenue Service building in Washington. / Susan Walsh, AP

by John Waggoner, USA TODAY

by John Waggoner, USA TODAY

Here's another reason to be annoyed at the 16-day government shutdown: It could delay tax refunds for millions of early filers by one or two weeks, the Internal Revenue Service announced Tuesday.

The IRS won't start processing returns until at least Jan. 28 and possibly as late as Feb. 4, instead of the planned start date of January 21. Should the government shut down again in January over budget talks scheduled as part of the deal to reopen the federal government, refunds could be tied up even longer.

Taxpayers still must file a return by April 15, and companies must still send W-2 and other tax forms on schedule, usually by Jan. 31.

"While H&R Block is well prepared to adjust to the IRS announcement of a delay in the opening of the 2014 tax season, we are frustrated for our clients who are among the estimated 18 million taxpayers who typically file a return in January," the tax preparation company said in a statement.

The IRS said it needs the extra time to program and test tax-processing systems. "The government closure came during the peak period for preparing IRS systems for the 2014 filing season," the IRS said in a release. "Programming, testing and deployment of more than 50 IRS systems is needed to handle processing of nearly 150 million tax returns."

About 90% of the IRS was idled during the partial government shutdown, putting the IRS about three weeks behind in preparations, the IRS said. "Readying our systems to handle the tax season is an intricate, detailed process, and we must take the time to get it right," acting IRS Commissioner Danny Werfel said.

During the closure, the IRS said, it received 400,000 pieces of correspondence, on top of the 1 million items already being processed before the shutdown.

Every year brings new changes and tweaks to the tax law, all of which have to be entered into the IRS computers.

"There are a lot of tax changes, particularly due to the 3.8% net investment income tax from the Affordable Care Act," said Melissa Labant,director of tax advocacy for American Institute of Certified Public Accountants. "The IRS has a lot of guidance they have to issue." That guidance, which goes out to tax preparers and individuals, also includes the matter of gay couples filing their federal returns as married couples.

"What concerns me is not the initial delay, but if there is any additional hiccup between now and January, the delays could be longer," Labant said.

The IRS will announce the date it will start to accept returns in December. There won't be any advantage to filing before that date. Electronic filers will still get their refunds more quickly than those who file by mail.

Companies will have to adhere to the same deadlines to get information to employees and investors.

This isn't the first time government gridlock has delayed the start of filing season. The 2013 tax filing season was delayed because Congress didn't pass laws pertaining to 2012 returns. And the IRS hotlines were closed for the Oct. 15 filing deadline for filers who requested extensions because of the most recent shutdown.